Running Problems - Mercruiser V8 5.0L

[Phuket44]

Hi All,

I live in Pattaya, Thailand. A few months ago a friend and I found a 20ft Glastron with 5.0L V8 inboard at an auction. I have owned outboards previously but first venture into inboards. Apparently the owner had bought a second boat and this one was left on the trailer for 5-6 years before he finally left the country and it was put up for sale. We picked it up very cheaply and were prepared for a bit of maintenance to get it ready for its return to the water. We had the compression checked by the local workshop guys and they said it was down on two cylinders, however when we got fuel into it (it was bone dry) and a new battery, we managed to start it and it ran and accelerated quite normally. After replacing the old throttle lever qaudrant and fitting a new fuel filter, we filled the tank and decided to put the thing in the water for a test run. Everything went well (even remembered to put the bung in) until we tried to accelerate. As soon as we tried to give it some throttle it would break down as if it was choking, and eventually backfired and sometimes died. Figuring that there may still be some bugs in it, we got onto Ebay and ordered a complete ignition set (leads plugs coil rotor, cap etc) which duly arrived. However, after fitting, same problem. We removed the carburettorand and after some surgery and a look over it, decided to opt for a fully reconditioned unit to eliminate this as a source of problems. This took some time to come but we finally fitted it, unfortunately same problem again. Runs like a dream on the trailer but breaks down under load in the water. I'm hoping someone has come accross a similar scenario and has some advice on where to go to next. We played with mixture settings, shift settings and timing. Any advice most welcome.

[pascavone]

run a temporary fuel tank and run it in the water. There is something blocking the fuel.

The backfire indicates a lean full mixture.

Also, the distributed has "advance weights" in it that could be stuck and not advancing the times at high RPM.

Could be as simple as a fuel pump or needle valve in the carburetor.

[cunuck]

check for a anti siphon valve between fuel tank and fuel pump.It has a ball and spring and can get sticky and dirty with old fuel .It is a cheep part some guys drill them out to get by but it should be replaced for safety it keeps fuel out of the bilge should your fuel line crack.

[ccruser1]

If indeed it has low compression on two cylinders that is very likely the source of your under power problems. It could run ok without a load but once you put a load on it she can't handle it because there isn't enough power to move it through the water.

good luck